CID 4764

C₂₀H₁₄O₄

Phenolphthalein

C 20
H 14
O 4

Ball-and-stick Stick Space-filling

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38 atoms, 41 bonds.

IUPAC name: 3,3-bis(4-hydroxyphenyl)-2-benzofuran-1-one
Molar mass: 318.328(47) g/mol
Atoms: 38
SMILES: C1=CC=C2C(=C1)C(=O)OC2(C3=CC=C(C=C3)O)C4=CC=C(C=C4)O
InChIKey: KJFMBFZCATUALV-UHFFFAOYSA-N
PubChem CID: 4764

Made of

C H O

Structural formula of phenolphthalein. Each unlabelled corner is a carbon atom, and the 12 hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

A solution holding phenolphthalein stays colourless while it is acidic, neutral or only slightly alkaline, and turns pink, then red, as it grows more strongly alkaline. Chemically it is a phenol, a compound with hydroxyl groups on benzene rings. In the ChEBI (Chemical Entities of Biological Interest) classification it is related to 2-benzofuran-1(3H)-one, which supplies its five-membered ring and the benzene ring fused to it. The IUPAC name, 3,3-bis(4-hydroxyphenyl)-2-benzofuran-1-one, records both parts, and the formula C₂₀H₁₄O₄ uses only carbon, hydrogen and oxygen, for a molar mass of 318.3 g/mol.

The solid is a fine crystalline powder, white to pale orange and sometimes yellowish-white, with neither smell nor taste. It melts at 262.5 °C. Water hardly dissolves it, while dilute alkali hydroxide solutions do, and it is very soluble in ethanol and acetone. It is made from phenol and phthalic anhydride, and the monosodium salt is the form most often sold, according to the Hazardous Substances Data Bank (HSDB).

Structure and bonding

Phenolphthalein’s 38 atoms form four rings in three groups, joined at one carbon. The hub sits in a five-membered ring with a ring oxygen, a C=O carbon and two carbons of a benzene ring. Together the two rings form the benzofuranone of the name. Its other two bonds lead to benzene rings, each tipped with an O–H group at its far end. Round the hub the angles run from 100° to 117° in PubChem’s computed model, uneven for a carbon with four neighbours.

Turned edge-on, each ring in the model is flat, the five-membered one in line with its fused benzene ring. In the Space-filling view the hub carbon nearly vanishes among its neighbours. With Labels on, four atoms read O: two in O–H groups, one in the five-membered ring and one in its C=O.

Each benzene ring is drawn as alternating single and double bonds, yet in the computed model all their C–C bonds measure 1.38 to 1.40 Å. Their double-bond electrons are shared round each ring, as in benzene. The model shows the neutral, colourless molecule, not the red form phenolphthalein takes in alkali after giving up protons.

Where it turns up

In the laboratory it is an acid–base indicator for titrating mineral and organic acids and most alkalis. For a strong acid against a strong base almost any indicator works, yet phenolphthalein is the usual choice, in Chem1’s account. Base goes in until one drop leaves the solution pink for good. In the ammonia fountain demonstration the water carrying it turns pink as it emerges and becomes alkaline.

Beyond the titration flask, it shows how deep carbonation has reached into concrete, paved surfaces included, a depth that signals when corrosion begins. Forensic work has used it to test for dilute blood, though it proved less specific than other reagents and reduced the DNA in a sample. It was once a test for bleeding in the gut. Chocolate and chewing-gum laxatives containing it were formerly sold worldwide, and it has turned up undeclared in several slimming products sold as dietary supplements.

Why it matters

A titration needs a signal at the equivalence point, where the added base has just used up the acid. An indicator supplies it as a second acid–base system in the flask, whose protonated and deprotonated forms differ in colour. It is chosen so that its pK_a, the figure that rates it as an acid, lies close to the pH expected at that point. Phenolphthalein’s pK_a is 9.7 at 25 °C on HSDB’s figure. Like any indicator, it changes colour over a range of pH rather than sharply at one value.

The same compound was once a laxative, and 20 manufacturers made laxatives containing it in 1997. That year the US Food and Drug Administration (FDA) proposed reclassifying it in over-the-counter laxatives. In 1999 the FDA stopped regarding it as generally recognised as safe and effective in stimulant laxatives. It was retired from that use because it was suspected of causing cancer, on the US National Cancer Institute’s account. The urine of people taking it could turn pink or red if alkaline enough.

Properties

Properties of phenolphthalein Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 4764.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₂₀H₁₄O₄
Molar mass	318.328(47) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	20
Hydrogen atoms	14
Oxygen atoms	4
Bonds	41
Single bonds	31
Double bonds	10
PubChem molecular weight	318.3 g/mol	g/mol	PubChem

Safety

In the European Union phenolphthalein carries the signal word Danger as a substance that may cause cancer. Its harmonised classification also calls it suspected of causing genetic defects and of damaging fertility. It may irritate the skin, eyes and digestive tract. The PubChem Laboratory Chemical Safety Summary for phenolphthalein gathers these classifications.

Questions

At what pH does phenolphthalein turn pink?

Its working range is pH 8.2 to 9.8 on Haz-Map's figure. In CAMEO Chemicals' description it is colourless up to pH 8.5 and pink to deep red above pH 9, then colourless again in a large excess of alkali. The colour also fades in sodium carbonate solution and saturated lime water, where the dye breaks down into phenol and 2-(4-hydroxybenzoyl)benzoic acid, according to the Hazardous Substances Data Bank.

What does a phenolphthalein molecule look like?

PubChem's 3D model draws 38 atoms, 20 of carbon, 14 of hydrogen and 4 of oxygen, held by 41 bonds. No single shape word fits, since the molecule has many centres rather than one central atom. Its three ring systems meet at one carbon bonded to four neighbours: two phenol rings and a fused pair of rings branch off from it in three directions. The model shows the neutral, colourless form.

How did phenolphthalein work as a laxative?

It stimulated the lining of the intestine and made its smooth muscle contract, in the US National Cancer Institute's description. In the Hazardous Substances Data Bank's account, it raised cyclic adenosine monophosphate in colon cells, which may have changed what those cells let through, so that fluid gathered. In rats it drew fluid into the colon, apparently by boosting the colon's prostaglandin E. It also cut water absorption in both intestines. A gut-liver cycle may have prolonged the effect.

Does phenolphthalein cause cancer?

Given by mouth, it caused tumours at several sites in mice and rats, and it damaged genetic material in several mammalian test systems. On that animal evidence, the US National Toxicology Program has listed it since 2000 as reasonably anticipated to cause cancer in people. Human studies are too limited to judge. In rodents it caused oxidative stress, a surplus of reactive free radicals, and disturbed tumour-suppressing genes, both believed to play a part in human cancer.

Made of the same elements

C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.